1. What should the API for PutMetricData look like?
2. Where should we store that data?
(you know, minor issues)
At the time, we decided that #1 was more critical than #2 (and of course #2 depends deeply on the answer to #1), and that the best way to solve it would be to do some learning-by-prototyping - stand up an internal version of CloudWatch to dogfood with some other Amazon services and be able to go to prospective internal customers with a "working" service that they could kick the tires on, not just some API docs and paper prototypes. So I quickly hacked up an implementation of "GraniteDataStore" called "InMemoryDataStore", which would store every PutMetricData request in a HashMap and nowhere else.
When I sent the code out for peer review, my reviewer was aghast at the idea that we would stand up a service that would knowingly store different copies of the data on every server, and lose it all on process restart. I responded by renaming InMemoryDataStore to FakeInMemoryDataStore, but that wasn't enough. Finally, we settled on a name of (if memory serves) FakeFakeFakeDataStoreThatLosesAllYourData and that was enough to get my merge approved.